Method for Constructing a Dipole Antenna

ABSTRACT

A method of fabricating a microwave antenna assembly is disclosed. The fabrication method includes providing a proximal portion having an inner conductor and an outer conductor, the inner conductor extending at least partially therein. The method further includes providing a distal portion disposed distally of the proximal portion, with the inner conductor extending at least partially therein. A high strength material may be injected from an inflow slot to an outflow slot of the distal portion such that the material is disposed in-between the inner conductor and a ceramic layer. The material bonds the distal portion and the ceramic layer to the proximal portion while providing mechanical strength to the distal portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 12/556,238, filed Sep. 9, 2009 and entitled “Method for Constructing a Dipole Antenna,” the contents of which are hereby incorporated by reference herein in their entirety.

BACKGROUND

1. Technical Field

The present disclosure relates generally to microwave applicators used in tissue ablation procedures. More particularly, the present disclosure is directed to a microwave applicator having either a liquid or solid loaded tip dipole antenna.

2. Background of Related Art

Treatment of certain diseases requires destruction of malignant tissue growths (e.g., tumors). It is known that tumor cells denature at elevated temperatures that are slightly lower than temperatures injurious to surrounding healthy cells. Therefore, known treatment methods, such as hyperthermia therapy, heat tumor cells to temperatures above 41° C., while maintaining adjacent healthy cells at lower temperatures to avoid irreversible cell damage. Such methods involve applying electromagnetic radiation to heat tissue and include ablation and coagulation of tissue. In particular, microwave energy is used to coagulate and/or ablate tissue to denature or kill the cancerous cells.

Microwave energy is applied via microwave ablation antennas that penetrate tissue to reach tumors. There are several types of microwave antennas, such as monopole and dipole. In monopole and dipole antennas, microwave energy radiates perpendicularly from the axis of the conductor. A monopole antenna includes a single, elongated microwave conductor. Dipole antennas may have a coaxial construction including an inner conductor and an outer conductor separated by a dielectric portion. More specifically, dipole microwave antennas may have a long, thin inner conductor that extends along a longitudinal axis of the antenna and is surrounded by an outer conductor. In certain variations, a portion or portions of the outer conductor may be selectively removed to provide for more effective outward radiation of energy. This type of microwave antenna construction is typically referred to as a “leaky waveguide” or “leaky coaxial” antenna.

Conventional microwave antennas typically has a long, thin inner conductor which extends along the axis of the probe and is surrounded by a dielectric material and is further surrounded by an outer conductor around the dielectric material such that the outer conductor also extends along the axis of the probe. In another variation of the probe which provides for effective outward radiation of energy or heating, a portion or portions of the outer conductor can be selectively removed. This type of construction is typically referred to as a “leaky waveguide” or “leaky coaxial” antenna. Another variation on the microwave probe involves having the tip formed in a uniform spiral pattern, such as a helix, to provide the necessary configuration for effective radiation. This variation can be used to direct energy in a particular direction, e.g., perpendicular to the axis, in a forward direction (i.e., towards the distal end of the antenna), or a combination thereof.

Invasive procedures and devices have been developed in which a microwave antenna probe may be either inserted directly into a point of treatment via a normal body orifice or percutaneously inserted. Such invasive procedures and devices potentially provide better temperature control of the tissue being treated. Because of the small difference between the temperature required for denaturing malignant cells and the temperature injurious to healthy cells, a known heating pattern and predictable temperature control is important so that heating is confined to the tissue to be treated. For instance, hyperthermia treatment at the threshold temperature of about 41.5° C. generally has little effect on most malignant growths of cells. However, at slightly elevated temperatures above the approximate range of 43° C. to 45° C., thermal damage to most types of normal cells is routinely observed; accordingly, great care must be taken not to exceed these temperatures in healthy tissue.

However, many types of malignancies are difficult to reach and treat using non-invasive techniques or by using invasive antenna probes designed to be inserted into a normal body orifice, i.e., a body opening which is easily accessible. These types of conventional probes may be more flexible and may also avoid the need to separately sterilize the probe; however, they are structurally weak and typically require the use of an introducer or catheter to gain access to within the body. Further, the manufacturing techniques for the conventional probe tend to be cumbersome, time consuming, and prohibitively expensive. Moreover, the addition of introducers and catheters necessarily increase the diameter of the incision or access opening into the body thereby making the use of such probes more invasive and further increasing the probability of any complications that may arise.

SUMMARY

A method of fabricating a microwave antenna assembly, which is structurally robust enough for unaided direct insertion into tissue is described herein. The microwave antenna assembly is generally comprised of a radiating portion which may be connected to a feedline (or shaft) which in turn may be connected by a cable to a power generating source such as a generator. The microwave assembly may be a monopole microwave antenna assembly but is preferably a dipole assembly. The distal portion of the radiating portion preferably has a tapered end which terminates at a tip to allow for the direct insertion into tissue with minimal resistance. The proximal portion is located proximally of the distal portion.

The adequate rigidity necessary for unaided direct insertion of the antenna assembly into tissue, e.g., percutaneously, preferably comes in part by a variety of different methods. A method of fabricating an antenna includes providing a proximal portion having an inner conductor and an outer conductor, the inner conductor extending at least partially therein. The method further includes providing a distal portion disposed distally of the proximal portion, with the inner conductor extending at least partially therein. A high strength polyimide material may be injected from an inflow slot to an outflow slot of the distal portion such that the polyimide material is disposed in-between the inner conductor and a ceramic layer. The polyimide material bonds the distal portion and the ceramic layer to the proximal portion while providing mechanical strength to the distal portion.

To further aid in strengthening the antenna assemblies the inner conductor may be affixed within the distal radiating portion in a variety of ways, for instance, welding, brazing, soldering, or through the use of adhesives. Forcing the inner conductor into a tensile condition helps to force the outer diameter of the antenna into a compressive state. This bi-directional stress state in turn aids in rigidizing the antenna assembly.

To enable a compressive state to exist near the outer diameter of the distal portion, a ceramic layer may be bonded to the polyimide material. Materials such as ceramic generally have mechanical properties where fracturing or cracking in the material is more likely to occur under tensile loading conditions. Accordingly, placing the distal portion under pre-stressed conditions, may aid in preventing mechanical failure of the distal portion if the antenna were to incur bending moments during insertion into tissue which could subject the distal portion under tensile loads. The ceramic layer and a coolant jacket also act as a dielectric buffer for aiding in keeping the efficiency of the antenna constant event though tissue is changing.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the following detailed description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a microwave ablation system according to an embodiment of the present disclosure;

FIG. 2 is an isometric view of a microwave antenna assembly according to the present disclosure;

FIG. 3 is an enlarged, cross-sectional view of a portion of the microwave antenna assembly of FIG. 2;

FIG. 4 is an enlarged, cross-sectional view of a portion of the microwave antenna assembly of FIG. 2;

FIG. 5 is a side view of a distal portion of a feedline of the microwave antenna assembly of FIG. 2;

FIG. 6 is an exploded view of the microwave antenna assembly according to the present disclosure;

FIGS. 7A-7C are enlarged cross-sectional views of sections A-A, B-B, and C-C of the microwave antenna assembly of FIG. 4;

FIG. 8 is a schematic diagram of a mold according to the present disclosure; and

FIG. 9 is a side view of another tip of the microwave assembly of FIG. 2.

DETAILED DESCRIPTION

Particular embodiments of the present disclosure will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail.

FIG. 1 shows a microwave ablation system 10 that includes a microwave antenna assembly 12 coupled to a microwave generator 14 via a flexible coaxial cable 16. The microwave antenna assembly 12 may be a dipole antenna of 1.6 cm in length. In order to ablate small tumors, microwave antenna assembly 12 has a Short Radiating Section (SRS). The microwave antenna assembly 12 is capable of reducing the antenna length to one-quarter of the wavelength length required, effectively using half the length of the half wave length dipole antenna. The generator 14 is configured to provide microwave energy at an operational frequency from about 500 MHz to about 5000 MHz.

Antenna assembly 12 is generally comprised of radiating portion 18, which may be connected by feedline 20 (or shaft) to the cable 16. More specifically, the antenna assembly 12 is coupled to the cable 16 through a connection hub 22. The connection hub 22 also includes an outlet fluid port 30 and an inlet fluid port 32 defined therein that are in fluid communication with a coolant jacket 38 and flow channel 13 (see FIG. 4). The coolant jacket 38 encloses a proximal portion 42 and the feedline 20 allowing coolant fluid from the ports 30 and 32 to be supplied and circulated around a portion of the antenna assembly 12. The ports 30 and 32 also include inner lumens defined therein (not shown) that are in fluid communication with the flow channel 13. The ports 30 and 32 are coupled to a supply pump 34 that is, in turn, coupled to a supply tank 36. The supply tank 36 stores the coolant fluid and maintains the fluid at a predetermined temperature. In one embodiment, the supply tank 36 may include a coolant unit which cools the returning liquid from the antenna assembly 12. In another embodiment, the coolant fluid may be a gas and/or a mixture of fluid and gas.

Assembly 12 also includes a tip 48 having a tapered end 24 that terminates, in one embodiment, at a pointed end 26 to allow for insertion into tissue with minimal resistance at a distal end of the radiating portion 18. In those cases where the radiating portion 18 is inserted into a pre-existing opening, tip 48 may be rounded or flat.

FIG. 2 illustrates the radiating portion 18 of the antenna assembly 12 having an unbalanced dipole antenna 40. The dipole antenna 40 includes a proximal portion 42 and a distal portion 44 interconnected by an injection molded seal 46. The distal portion 44 and the proximal portion 42 are of different, unequal lengths so that the dipole antenna 40 is unbalanced. In one embodiment, the distal portion 44 may be longer than the proximal portion 42. In one embodiment, in which the feedline 20 is formed from a coaxial cable, the outer conductor 56 and the inner insulator 52 may be sliced off to reveal the inner conductor 50, as shown in FIG. 5.

The dipole antenna 40 is coupled to the feedline 20 that electrically connects antenna assembly 12 to the generator 14 (FIG. 1). The assembly 12 includes a coolant jacket 38 coupled to a fluid seal 8 (see FIG. 4), which in turn is coupled to an injection molded seal 46. The coolant jacket 38 may be formed from a medical grade metal. The injection molded seal 46 may be made of a high strength polyimide resin. The polyimide resin may be VESPEL® sold by DuPont of Wilmington, Del.

In one embodiment, the injection molded seal 46 is fabricated by injecting a polyimide material into an inflow slot 9A to an outflow slot 9B of the distal portion 44. As shown in FIG. 3, the injection molded seal 46 is disposed in-between a distal radiating section 44 and the ceramic layer 2. The ceramic layer may be made of alumina ceramic. The injection molded seal 46 bonds the distal portion 44 and the ceramic layer 2 to the proximal portion 42 while providing mechanical strength to the distal portion 44. As shown in FIGS. 3-4, the feedline 20 includes an inner conductor 50 (e.g., wire) surrounded by an inner insulator 52, which is then surrounded by an outer conductor 56 (e.g., cylindrical conducting sheath). The inner and outer conductors 50, 56 may be constructed of copper, gold, stainless steel or other conductive metals with similar conductivity values. The metals may be plated with other materials, e.g., other conductive materials, to improve their properties, e.g., to improve conductivity or decrease energy loss, etc. In one embodiment, the inner insulator layer 52 is formed from a fluoropolymer, such as tetrafluorethylene, perfluorpropylene, and the like, and has a thickness of about 0.011-0.013 inches.

In one embodiment, the feedline 20 may be formed from a coaxial semi-rigid or flexible cable having a wire with a 0.047″ outer diameter rated for 50 Ohms. The inner insulator 52 may have a dielectric constant from about 1 to 10.

Overlaying the outer conductor 56 is a flow channel 13 that cools the majority of the proximal portion 42. The flow channel 13 is in fluid communication with fluid ports 30, 32. A polyimide inflow sleeve 15 is disposed in the flow channel 13 to create an inflow channel 17 a and an outflow channel I 7 b for the coolant. The abundance of cooling fluid from the concentric in-flow design of the polyimide inflow sleeve 15 in the flow channel 13 acts as a lossy material to absorb the microwave energy as well as to cool the feedline 20 for percutaneous use.

In another embodiment, the fluid seal 8 may also be formed from solid wire machined component or a cylindrical conductor filled with solder. The fluid seal 8 is thereafter coupled to the outer conductor 56 (joint E), as shown in FIGS. 3-4. This may be accomplished by soldering the fluid seal 8 to the outer conductor 56, such as by melting the solder of the fluid seal 8 and inserting the outer conductor 56 therein.

The distal portion 44 includes a conductive member 45 that may be formed from any type of conductive material, such as metals (e.g., copper, stainless steel, tin, and various alloys thereof). The distal portion 44 may have a solid structure and may be formed from solid wire (e.g., 10 AWG). In another embodiment, the distal portion 44 may be formed from a hollow sleeve of an outer conductor of coaxial cable or another cylindrical conductor. The cylindrical conductor may then be filled with solder to convert the cylinder into a solid shaft. More specifically, the solder may be heated to a temperature sufficient to liquefy the solder within the cylindrical conductor (e.g., 500° F.), thereby creating a solid shaft.

As shown in FIGS. 2 and 3, the distal portion 44 is coupled to the tip 48, which may be formed from a variety of heat-resistant materials suitable for penetrating tissue, such as metals (e.g., stainless steel) and various thermoplastic materials, such as poletherimide, polyimide thermoplastic resins, an example of which is Ultem® sold by General Electric Co. of Fairfield, Conn. The tip 48 may be machined from various stock rods to obtain a desired shape. The tip 48 may be attached to the distal portion 44 using various adhesives, such as epoxy seal. If the tip 48 is metal, the tip 48 may be soldered to the distal portion 44 or may be machined as one continuous component.

FIG. 6 is an exploded view of the microwave antenna assembly 12. The microwave antenna assembly 12 includes a proximal portion 42 and a distal portion 44. The proximal portion 42 may include an inner conductor 50, an inner insulator layer 52, and an outer conductor 56. The proximal portion 42 may also include a flow channel 13 defined therein (not shown) that includes an inflow channel 17 a and an outflow channel 17 b that are separated by a polyimide inflow tube 15. The polyimide inflow tube 15 (not shown) may be inserted into a pocket of a fluid seal 8 to cool the proximal portion 42. The distal portion 44 includes an inner conductor 50, a distal radiating section 51, an injection molded seal 46, and a ceramic layer 2.

FIGS. 7A-7C are enlarged cross-sectional views of sections A-A, B-B, and C-C of the microwave antenna assembly of FIG. 4. FIG. 7A illustrates a cross section at section A-A. Section A-A illustrates from the inside towards the outer surface, an inner conductor 50, insulator 52, outer conductor 56, flow channel 13 (specifically inflow channel 17 a), polyimide inflow tube 15, flow channel 13 (specifically outflow channel 17 b), and coolant jacket 38.

FIG. 7B illustrates a cross section at section B-B. Section B-B illustrates from the inside towards the outer surface, an inner conductor 50, insulator 52, outer conductor 56, flow channel 13 (specifically inflow channel 17 a), polyimide inflow tube 15, flow channel 13 (specifically outflow channel 17 b), fluid seal 8, and coolant jacket 38. FIG. 7C illustrates a cross section taken at section C-C of the distal portion. Section C-C illustrates from the inside towards the outer surface, an inner conductor 50, insulator 52, an injection molded seal 46, and a ceramic layer 2.

Referring back to FIGS. 2-4, the microwave antenna assembly 12 may be manufactured in various steps. A coaxial cable that includes the inner conductor 50, insulator layer 52, and outer conductor 56 may be manufactured and assembled as one component. The outer conductor 56 may be soldered to the fluid seal 8, for example at joint E, to provide the electrical joint, if needed. The coolant jacket 38 may be bonded, threaded, laser welded, soldered or crimped to the fluid seal 8 at joint D. The coolant jacket 38 and the fluid seal 8 may be assembled as one component. The polyimide inflow tube 15 may be configurable to slide into a pocket of the fluid seal 8.

The inner conductor 50 is configured to slide inside a hole of the distal portion 44. The distal portion 44 is affixed to a distal end of the inner conductor 50 by laser welding, soldering or crimping at joint F. The coaxial cable, coolant jacket 38, proximal portion 42, ceramic layer 2, and distal portion 44 are placed into an injection mold cavity.

FIG. 8 is a schematic diagram of a mold according to the present disclosure. The mold 53 is used to inject a high strength polyimide material in-between the inner conductor 50 and the ceramic layer 2. The mold 53 includes mold halves 111 a and 111 b. Mold halves 111 a and 111 b include portions/cavities to receive coaxial cable, coolant jacket 38, fluid seal 8, ceramic layer 2, and trocar tip 48. Mold halves 111 a and 111 b also include an inflow slot 9A and an outflow slot 9B. The mold halves 111 a and 111 b are clamped tightly together and heated polyimide is injected into the inflow slot 9A until the heated polyimide fills into outflow slot 9B defined therein. The polyimide flows into a cavity to form a uniform layer of polyimide layer along the distal portion 44. The polyimide material bonds the distal portion 44 and the ceramic layer to the proximal portion while providing mechanical strength to the distal portion.

In another embodiment, the mold 53 does not include a cavity for the trocar tip 48. In such an embodiment, when the injection molding process is complete, the antenna assembly 12 is finished by installing the trocar tip 48. FIG. 9 illustrates various shapes and forms of a trocar tip 48 installed onto a sheath 38, namely a stainless steel tip 48 a and a dielectric tip 48 b. Both tips 48 a and 48 b include insertion bases 51 a and 51 b having an external diameter that is smaller than diameter of the tips 48 a and 49 allowing for easier insertion into a sheath 38. The configuration also provides for a better seal between the tip 48 and the sheath 38. In another embodiment, the sheath 38 and tip 48 c maybe threaded so as to attach to each other. Therefore, the tip 48 c may be tightly screwed into the sheath 38.

The described embodiments of the present disclosure are intended to be illustrative rather than restrictive, and are not intended to represent every embodiment of the present disclosure. Various modifications and variations can be made without departing from the spirit or scope of the disclosure as set forth in the following claims both literally and in equivalents recognized in law. 

What is claimed is:
 1. A method of constructing a dipole antenna, comprising the steps of: providing an upper mold half and a lower mold half having mating surfaces that mate together to form cavities to receive a feedline, radiating portion, and ceramic portions therein, each of the upper and lower mold halves includes first and second slots, respectively, to receive a polyimide material; placing the feedline into the respective cavity, the feedline including an inner conductor, an outer conductor and an inner insulator disposed therebetween; placing the radiating portion into the respective cavity, the radiating portion including an unbalanced dipole antenna having a proximal portion and a distal portion of different lengths, wherein the proximal portion includes at least a portion of the inner conductor and the inner insulator and the distal portion includes a conductive member; placing the ceramic portions into the respective cavity; mating the upper mold half and the lower mold half together; and depositing the polyimide material into each of the first slots through the second slots and upon cooling, the polyimide material adheres to the distal and ceramic portions to the proximal portion.
 2. The method of constructing the dipole antenna in accordance with claim 1, wherein the distal portion is affixed to a distal end of the inner conductor by at least one of laser welding, soldering, and crimping.
 3. The method of constructing the dipole antenna in accordance with claim 1, further comprising the step of joining a trocar adapted to receive a distal end of the inner conductor. 